The "apparent ease" with which resistance emerged in H7N9 viruses to the only available treatment is a cause for concern, said the Chinese researchers who conducted the study.
On Monday, Chinese health authorities said no new human cases of the deadly H7N9 bird flu strain had been recorded for the second week in a row.
A total of 130 people have been infected, and 37 died.
For the latest study, researchers followed 14 H7N9 patients admitted to the Shanghai Public Health Clinical Centre in China between April 4 and 20.
Three, however, had a "persistently high viral load" in spite of treatment and had to be placed on respiratory support machines.
Two of them died, said the study authors.